Surface modification by the deposition of metals on a variety of substrate materials is a known technique in many fields, such as for example, in the production of printed circuit boards, in plating automobile parts, in plating fittings, etc. The deposited metal layers change the surface characteristics of the coated substrates in decorative and/or functional respects. Electrolytic and electroless metal deposition methods are known for depositing metals.
In either electrolytic or electroless deposition methods, the substrate to be coated is contacted with a composition comprising deposition metal ions. In electrolytic plating, metal deposition occurs when an applied current reduces the deposition metal ions to metal on the surface of the substrate, which acts as a cathode. In electroless deposition, metal deposition occurs when the deposition metal ions are reduced to metal by a redox reaction with a reducing agent.
In either method, the plating composition changes during the course of deposition. For example, the concentration of deposition metal ions decreases. During an electroless plating operation, the concentration of the reducing agent also decreases. Moreover, the concentration of by-products from the plating process, such as by the oxidation of the reducing agent in electroless plating or the oxidation of bath additives at the anode in electrolytic plating, increases. To assure relatively constant performance of the plating bath throughout a plating operation, deposition metal ions may be replenished by adding the metal ion, as a soluble salt, to the plating composition. In electroless plating, the reducing agent, also typically as a soluble salt, may be replenished. Finally, methods have been developed to remove oxidation products from the plating composition, such as by, for example, anion exchange.
Since the deposition metal ions are typically added as soluble salts, replenishment of the plating composition causes the concentration of anions in the plating composition to increase. It is known that increased anion concentration adversely affects the quality of the plating composition and the plated metal. The efficiency of a plating composition may be measured by the number of “metal-turnovers” (MTO) before bath quality and/or deposited metal quality are compromised. One MTO refers to one complete conversion of the original deposition metal ion concentration. For example, for a plating composition having an original metal ion concentration of 1.0 M, one MTO is completed when 1.0 M of the deposition metal ion has been deposited and replenished in the plating bath. Since replenishment of deposition metal ion by the addition of soluble salts increases the concentration of anions in the plating composition, a typical plating composition may achieve between two and five MTO before the plating bath and/or plated metal quality are compromised by an unacceptably high concentration of anion.
Methods have been developed to control the anion concentration by removing oxidation products. For example, DE 19851180 discloses a method for replenishing hypophosphite ion, which is a reducing agent, in an electroless plating composition using a weak base anion exchange membrane. Orthophosphite ion, present in the plating composition as a by-product of the hypophosphite reduction process, can be removed with simultaneous replenishment of hypophosphite reducing agent.
U.S. Pat. No. 2,726,968 also discloses the use of an anion exchange membrane in an electroless nickel plating bath for the replenishment of hypophosphite reducing agent. In this method, hypophosphite is added to the bath while phosphite ion by-product is removed.
DE 4310366 discloses a method for replenishing an electroless plating composition with hypophosphite reducing agent. In this method, an anion exchange membrane is coupled with electrolysis. Hypophosphite and orthophosphite are removed from the electroless plating bath using the anion exchange membrane. The orthophosphite byproduct can be reduced to the hypophosphite reducing agent by electrolysis. After electrolysis, the hypophosphite is returned to the electroless plating bath.